Fuel injector

ABSTRACT

A conventional fuel injection system meters fuel by varying the time for which the injector is open. This is an empirical rather than fundamental method of metering the fuel. The amount of fuel delivered for a given injector opening time will depend on the pressure difference across the injector orifice and the exact dimensions of the injector orifice. Both these parameters are subject to variation. An injector ( 1 ) which comprises a plunger ( 2 ), a solenoid coil ( 3 ), and a conical return spring ( 11 ) which is driving a primary high pressure pump assembly consisting of a primary piston ( 4 ) within a primary cylinder ( 5 ) together with primary inlet valve ( 8 ) and primary outlet valve ( 7 ). The plunger ( 2 ) is also driving a secondary low pressure pump assembly consisting of a secondary piston ( 22 ), secondary cylinder ( 23 ), secondary inlet valve ( 24 ) and secondary outlet valve ( 25 ). The solenoid coil ( 3 ) is under the control of an electronic monitor unit ( 10 ) which is connected to the solenoid coil ( 3 ) via two connecting wires ( 19 ). The injector is mounted in and is delivering fuel into an inlet manifold ( 20 ). By interrogating various sensors around the engine the electronic monitor unit ( 10 ) calculates the amount of fuel it wishes to deliver. From a simple volumetric calculation the electronic monitor unit ( 10 ) calculates how far the piston ( 4 ) must be moved to deliver this amount of fuel.

This is a United States national stage application of Internationalapplication No. PCT/GB01/00302, filed Jan. 26, 2001, the benefit of thefiling date of which is hereby claimed under 35 U.S.C. § 120, which inturn claims the benefit of United Kingdom application No. 0001751.7,filed Jan. 27, 2000, and United Kingdom application No. 0016408.7, filedJul. 5, 2000, the benefit of the filing date of which is hereby claimedunder 35 U.S.C. § 119.

The present invention relates to a fuel injector and is concernedparticularly with a fuel injector comprising an electronicallycontrolled fuel injection system for a gasoline engines.

At present fuel injectors are used to inject a fuel spray into inlet amanifold or a cylinder of an engine. These types of injectors must becapable of two basic functions. Firstly they must be able to deliver ametered amount of fuel for each inlet stroke of the engine so that asuitable air fuel ratio can be maintained. Secondly they must atomisethe fuel by forcing it through a small orifice at high pressure toimprove the efficiency of the combustion process.

Conventional fuel injection systems use fuel injectors comprise a simplesolenoid controlled valves. These valves are not proportional but areeither fully open or fully closed. The injectors are linked to anexternal high pressure fuel supply consisting of a fuel pump, highpressure fuel lines and a pressure regulator. The fuel pressure istypically between 2 and 4 bar. The fuel is metered by varying the timefor which the injectors are open. The fuel is atomised by forcing itthrough a precisely dimensioned small orifice in the injectors.

The external high pressure fuel system is expensive and bulky. It alsopresents a potential safety hazard as any breach in the fuel lines willresult in a rapidly expanding fuel mist which is extremely flammable.The present invention does not require a high pressure fuelling systemand thus reduces both system cost and risk.

A conventional fuel injection system meters fuel by varying the time forwhich the injector is open. This is an empirical rather than fundamentalmethod of metering the fuel. The amount of fuel delivered for a giveninjector opening time will depend on the pressure difference across theinjector orifice and the exact dimensions of the injector orifice. Boththese parameters are subject to variation. The pressure difference willvary due To both the fuel pressure varying on the inside of the injectorand the pressure in the inlet or cylinder varying on the outside of theinjector. The fuel pressure will vary in the short term with systemvoltage and pulses from the fuel pump, and in the long term as theregulator ages, and any fuel filters in the system clog up. The pressurein the inlet manifold or cylinder will vary with load and engine speed,and will also change as engine components wear. The effective dimensionsof the atomisation orifice may vary due to blockage or wear. All ofthese effects will result in an error in the amount of fuel deliveredfor a given injector opening time. The present invention meters theamount of fuel delivered from first principles using a simple volumetriccalculation and is thus not subject to these sources of error.

GB 2052794A discloses a fuel injector comprising a first solenoidconnected to a valve assembly, the first solenoid opening the valveassembly to dispense fuel from the valve assembly when an electriccurrent is applied to the first solenoid. A second solenoid is used tomeasure the distance the first solenoid moves and varies the currentflowing through the second solenoid to vary the amount of fueldispensed.

According to a first aspect of the invention there is provided a fuelinjector comprising an electrical actuator comprising a solenoid coiland a movable plunger; a pump assembly comprising piston means beingmovable by the electrical actuator; and monitoring means for controllingthe amount of fuel dispensed, the arrangement being such that in usefuel is dispensed from the fuel injector by the movement of the plungerand the piston means, and characterised in that the monitoring meansanalyses the change in the electrical properties of the solenoid coilcaused by the change in the position of the plunger within the solenoidcoil to ascertain the position of the plunger.

Preferably, the analysis of the position of the plunger is used bymonitoring means to control the amount of fuel being dispensed.

In use the movement of the plunger of the electrical actuator and thepiston means over a predetermined distance will preferably dispense apredetermined amount of fuel

The actuator preferably comprises a fixed solenoid coil and a returnspring for the movable plunger.

Preferably the monitoring means comprises position measurement circuitrythe arrangement being such that in use the solenoid coil provides motiveforce to the plunger and to the piston means, and the solenoid coilprovides positional feedback information of the position of the plungerto the position measurement circuitry of the monitoring means.

Preferably the solenoid coil comprises a single winding with twoexternal electrical connections.

The fuel injector preferably comprises fuel atomisation means, thearrangement being such that in use fuel is forced through theatomisation means by the movement of the actuator; the actuatorgenerating sufficient force to pressurise the fuel too a level whichwill obtain satisfactory atomisation.

Preferably, the atomisation means is an atomisation orifice.

Preferably the actuator comprises an end stop, the arrangement beingsuch that with no current flowing through the solenoid coil the returnspring moves the plunger in a direction towards the end stop; this endstop being referred to as the current off end stop.

Preferably when current is passed through the solenoid coil the plungermoves in a direction away from the current off end stop and the pistonmeans dispenses fuel from the fuel injector.

In use during normal operation the monitoring means preferably provideselectrical power to the solenoid coil and energises the solenoid coilfor short periods of Time such that the distance the plunger moves awayfrom the current off end is controlled by the duration and level of thepulse of electrical current that is applied to the solenoid coil.

Preferably the monitoring means controls and varies the amount of fueldispensed by varying the duration of the pulse of electrical currentthat is applied to the solenoid coil, the arrangement being such that inuse the greater the duration of the current pulse that is applied to thesolenoid coil the greater the distance moved by the plunger and thepiston means and the greater the amount of fuel dispensed.

The monitoring means preferably controls and varies the amount of fueldispensed by varying the level of current that is passed through thesolenoid coil during a current pulse, the arrangement being such that inuse the higher the current level that is applied during the currentpulse the greater the distance moved by the plunger and the piston meansand the greater the mount of fuel dispensed.

Preferably the monitoring means controls the amount of fuel dispensed byvarying both the duration of the current pulse and the level of currentwithin that pulse.

Preferably in use the monitoring means applies a current pulse of apredetermined duration and level to the solenoid coil and once thecurrent pulse has ended the monitoring means monitors the position ofthe plunger as it continues to move in the same direction due to its ownmomentum, and as the plunger slows to a halt due to the forces exertedupon it by the return spring and the fuel pressure on the end of thepiston means, the monitoring means notes the maximum deflection reachedby the plunger.

Preferably the monitoring means uses the measurement of the maximumdeflection reached by the plunger to establish an empirical relationshipbetween the duration and level of the current pulse, and the distancemoved by the plunger.

The monitoring means preferably uses the empirically derivedrelationship between the duration and level of the current pulse, andthe distance moved by the plunger, to predict the duration and level ofthe current pulse that needs to be applied to the solenoid coil to movethe plunger the distance that will dispense a required amount of fuelfrom the fuel injector.

Preferably in use for any given fuel dispensing stroke of the pistonmeans the monitoring means notes the actual distance moved by theplunger and then compares this distance to a predetermined distance theplunger was required to move in order to deliver predetermined correctamount of fuel; the difference between the predetermined and actualdistance provides an error term that is fed into a feedback algorithmthat modifies the empirical relationship between the duration and levelof the current pulse, and the distance moved by the plunger. Thisfeedback mechanism allows for drift in the performance characteristicsof the injector as may be caused by temperature, wear, changes in returnspring characteristics, or changes in fuel viscosity.

Preferably when no current is flowing in the solenoid coil and thereturn spring has returned the plunger to the current off end stop theoutput from the positional feedback system will be noted and used as anoffset reference figure.

Preferably during a fuel dispensing operation the offset referencefigure obtained when the plunger is resting on the current off end stopis subtracted from all subsequent positional feedback system outputreadings to enable the distance moved by the plunger from the currentoff end stop to be calculated. This means that the absolute value of thepositional feedback system output is not important, rather it is thechange in the positional feedback system output that indicates theamount of fuel dispensed; this mechanism allows for mid to long termdrift in the positional feedback system output as might be caused bytemperature, wear or ageing.

The fuel injector preferably comprises a second end stop at the otherend of the travel of the plunger against, the arrangement being suchthat the plunger rests against the second end stop if a current ispassed through the solenoid coil for long periods of time. This isreferred to as the current on end stop; the mechanical arrangement beingtightly toleranced so that the distance the plunger can move between thecurrent off end stop and current on end stop is precisely known; thisenabling a positional feedback system calibration operation that iscarried out by noting the positional feedback system output with nocurrent flowing through the solenoid coil and plunger resting on thecurrent off end stop, and the positional feedback system output withcurrent flowing through the solenoid coil for a long period and theplunger resting on its current on end stop; the system will then havetwo positional feedback system output readings a known distance apartwhich will enable it to calculate the scaling factor of the positionalfeedback system.

Preferably for some applications during normal operation of an enginethe fuel injector is capable of delivering any required amount of fuelfor the engine with a stroke of the plunger and piston means that isless than the maximum stroke limit of the injector as dictated by thecurrent on end stop. The amount of fuel can be varied as required byvarying the level and or duration of the current pulse applied to thesolenoid coil without being limited by the current on end stop; as aresult the positional feedback system calibration operation will be notbe carried out during normal operation of the engine but will be carriedout when the system is commissioned and during routine serviceoperations.

Preferably for some applications the fuel dose will under somecircumstances where a higher than normal fuel dose is required, such ascold start, be delivered in one or more strokes, at least one of thosestrokes being over the full stroke distance of the plunger as dictatedby the current on end stop, this enabling the positional feedback systemcalibration operation to be carried out more frequently.

Preferably for some applications of the fuel injector delivers the fuelin two strokes of the plunger and piston means, at least one of thosestrokes being over the full stroke distance of the plunger as dictatedby the current on end stop, this enabling the positional feedback systemcalibration operation to be carried out more frequently.

Preferably for some applications the pump assembly comprises two pumps;a primary high pressure pump and a secondary low pressure pump, thearrangement being such that, in use, the primary high pressure pumptransfers the metered fuel dose into the engine and the secondary lowpressure pump generates positive pressure during an inlet stroke of theprimary high pressure pump reducing the tendency for the formation ofbubbles within the primary high pressure pump.

The primary high pressure pump preferably comprises a primary pistonreciprocatable within a primary cylinder chamber comprising a primaryinlet valve and a primary outlet valve.

The secondary low pressure pump preferably comprises a secondary pistonreciprocatable within a secondary cylinder chamber comprising asecondary inlet valve and a secondary outlet valve.

The primary piston and the secondary piston are preferably substantiallycoaxial.

Preferably the diameter of the primary piston is less than the diameterof the secondary piston.

Preferably for some applications the inlet stroke of the primary highpressure pump occurs during the outlet stroke of the secondary lowpressure pump; during the outlet stroke of the secondary low pressurepump the pressure of the fuel in the secondary cylinder chamber of thesecondary low pressure pump being raised to a value dictated by theopening pressure of the secondary outlet valve of the secondary lowpressure pump; the primary inlet valve of the primary high pressure pumpbeing in fluid communication with the cylinder chamber of the secondarylow pressure pump and the pressurised fuel therein.

In an embodiment of the present invention for a predetermined plungermovement the volume of fuel displaced by the secondary low pressure pumpis considerably greater than the volume of fuel displaced by the primaryhigh pressure pump; as a result during the inlet stroke of the primaryhigh pressure pump and outlet stroke of the secondary low pressure pumponly a part of the fuel being pumped by the secondary low pressure pumpis drawn into the primary high pressure pump; the excess fuel pumped bythe secondary low pressure pump passing through the secondary outletvalve of the secondary low pressure pump so allowing for the generationof net positive pressure during the inlet stroke of the primary highpressure pump as dictated by the opening pressure of the secondaryoutlet valve of the secondary low pressure pump.

Preferably for some applications of the fuel injector the inlet valve ofthe primary high pressure pump may be incorporated within the plunger.

According to a second aspect of the invention there is provided a methodfor detecting the position of a plunger of a fuel injector comprising anelectrical actuator comprising a solenoid coil and a movable plunger; apump assembly comprising piston means being movable by the electricalactuator and monitoring means for controlling the amount of fueldispensed, the method comprising first moving the plunger and the pistonmeans using the solenoid coil, and characterised in that the methodfurther comprises monitoring the change in the electrical properties ofthe solenoid coil caused by the change in the position of the plungerwithin the solenoid coil to ascertain the position of the plunger.

Preferably, the method comprises the step analysing the position of theplunger with the monitoring means and using the analysis for controllingThe amount of fuel being dispensed by the fuel injector.

The invention may include any combination of the features or limitationsreferred to herein.

The apparatus may be carried into practice in various ways, but anembodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional side elevation of a fuel injector with asolenoid coil de-energised and at the end of the return stroke; and

FIG. 2 is a cross-sectional side elevation of the fuel injector shown inFIG. 1 with the solenoid coil energised and at the end of the deliverystroke.

With reference to FIG. 1, an injector 1 which comprises a plunger 2, asolenoid coil 3, and a conical return spring 11 which is driving aprimary high pressure pump assembly consisting of a primary piston 4within a primary cylinder 5 together with primary inlet valve 8 andprimary outlet valve 7. The plunger 2 is also driving a secondary lowpressure pump assembly consisting of a secondary piston 22, secondarycylinder 23, secondary inlet valve 24 and secondary outlet valve 25. Thesolenoid coil 3 is under the control of an electronic monitor unit 10which is connected to the solenoid coil 3 via two connecting wires 19.The injector is mounted in and is delivering fuel into an inlet manifold20.

The primary piston 4 is linked directly to the plunger 2. The primarypiston 4 is precisely toleranced within the primary cylinder 5 toprevent fuel leaking between the primary piston 4 and primary cylinder 5when the fuel is pressurised. The primary cylinder 5 is made of amaterial which has a similar coefficient of expansion to the primarypiston 4 material to ensure a close tolerance is maintained over a widetemperature range. The primary piston 4 and primary cylinder 5 are madeof materials which when combined as a bearing pair have suitable wearcharacteristics to provide the necessary component life.

The secondary piston 22 is linked directly to the plunger 2. Thesecondary piston 22 is precisely toleranced within the secondarycylinder 23 to prevent fuel leaking between the secondary piston 22 andsecondary cylinder 23 when the fuel is pressurised. The secondarycylinder 23 is made of a material which has a similar coefficient ofexpansion to the secondary piston 22 material to ensure a closetolerance is maintained over a wide temperature range. The secondarypiston 22 and secondary cylinder 23 are made of materials which whencombined as a bearing pair have suitable wear characteristics to providethe necessary component life. The secondary piston 22 is of greaterdiameter than the primary piston 4 to ensure that for a given movementof the plunger 2 the secondary pump assembly will displace a greatervolume of fuel than the primary pump assembly. This ensures that apositive pressure can be generated within the secondary pressurisationchamber 26 during the inlet stroke of the primary high pressure pumpassembly.

The maximum travel of the plunger is limited by two end stops, thecurrent off end stop 15 and the current on end stop 16. When at restwith no current passing though the solenoid coil 3 the plunger 2 is heldagainst the current off end 15 stop by the return spring 11. FIG. 1shows the injector in this state. When the solenoid coil 3 ispermanently energised the plunger 2 is held against the current on endstop 16. FIG. 2 shows the injector in this state. The maximum distancethe plunger 2 can move between the two end stops is known and preciselytoleranced. This enables a self calibration operation to take place.This is described below.

During normal operation the plunger 2 will not move as far as thecurrent on end stop 16 as this would limit the amount of fuel that theinjector can deliver. The injector will be designed such that the volumeof fuel delivered by moving the plunger 2 from its current off end stop15 to its current on end stop 16 will be greater than the maximum fueldose required during normal operation.

Primary inlet valve 8 is a low mass fast response one way valveinstalled within the moving plunger. The valve shown consists of a balland compression spring but a reed valve or any other suitable design ofone way valve could be employed. The primary inlet valve 8 will openwhen the pressure of the fuel in the secondary pressurisation chamber 26exceeds the pressure of the fuel in the primary pressurisation chamber21 by the opening pressure of the primary inlet valve 8. A drillingwithin the primary piston 4 provides a path for the fuel to flow fromthe secondary pressurisation chamber 26 to the primary pressurisationchamber 21. The primary inlet valve 8 is designed to minimise thepressure required for it to open.

Primary outlet valve 7 is a low mass fast response one way valve. Thevalve shown consists of a ball and compression spring but a reed valveor any other suitable design of one way valve could be employed. Theprimary outlet valve 7 will open when the pressure of the fuel in theprimary pressurisation chamber 21 exceeds the pressure in theatomisation valve chamber 12 by the opening pressure of the primaryoutlet valve 7.

The atomisation valve consists of an atomisation pintle valve 13 heldclosed by a atomisation valve spring 14. When the pressure of the fuelin the atomisation valve chamber 12 rises lo a level that exerts enoughforce on the atomisation pintle valve 13 to compress the atomisationvalve spring 14 the valve will open. The advantage of this atomisationmechanism is that the atomisation pressure will be constant. Theatomisation valve spring 14 compression force would normally be set togive an atomisation pressure of at least 3 bar.

Secondary inlet valve 24 is a low mass fast response one way valve. Thevalve shown consists of a ball and compression spring but a reed valveor any other suitable design of one way valve could be employed. Thesecondary inlet valve 24 will open when the pressure of the fuel in thefuel inlet 17 exceeds the pressure of the fuel in the secondarypressurisation chamber 26 by the opening pressure of the secondary inletvalve 24. The secondary inlet valve 24 is designed to minimise thepressure required for it to open.

Secondary outlet valve 25 is a low mass fast response one way valve. Thevalve shown consists of a ball and compression spring but a reed valveor any other suitable design of one way valve could be employed. Thesecondary outlet valve 25 will open when the pressure of the fuel in thesecondary pressurisation chamber 26 exceeds that in the end stop chamber6 by the opening pressure of the secondary outlet valve 25. The openingpressure of the secondary outlet valve 25 dictates the pressure in thesecondary pressurisation chamber 26 during the inlet stroke of theprimary high pressure pump assembly. It will be set at a value thatensures no bubbles form within the fuel during this phase of operation.

The injector is also provided with a positional feedback system. Thisconsists of position measurement circuitry within the monitor unit 10which utilises the change in electrical characteristics of the solenoidcoil 3 as the plunger 2 moves within it.

Fuel is supplied into the main body of the injector by a fuel inlet 17.A fuel outlet 18 provides a return path to the fuel tank. As the plungerreciprocates the secondary low pressure pump assembly circulates fuelthrough the injector and back to the tank, a proportion of this fuelbeing drawn off by the primary high pressure pump assembly for injectioninto the engine. A path for the excess fuel is provided through thesecondary outlet valve 25, the end stop chamber 6, between the plunger 2and the inside of the solenoid coil 3, and finally around the outside ofthe solenoid coil 3 to the fuel outlet 18. The plunger 2 and solenoidcoil 3 are thus immersed in circulating fuel which helps to carry awayheat from these components.

The end stop chamber 6 is full of fuel. When the plunger 2 makes contactwith the current off end stop 15 or the current on end stop 16 the fuelwill act as a damper to prevent the plunger 2 bouncing. It will alsodampen out any mechanical noise.

The injector 1 operates as follows. By interrogating various sensorsaround the engine the electronic monitor unit 10 calculates the amountof fuel it wishes to deliver. From a simple volumetric calculation theelectronic monitor unit 10 calculates how far the piston 4 must be movedto deliver this amount of fuel. At the start of the injection operationthe plunger 2 will be held against its current off end stop 15 by thereturn spring 11. In this state all the pump valves will be closed. Fromprevious fuel injection operations the electronic monitor unit 10 willhave built up an empirical relationship between solenoid coil currentpulse duration and plunger movement. This relationship will take theform of a programmable lookup table or similar. The empiricalrelationship is used to predict the length of time the solenoid coil 3must be energised to displace the plunger 2 and the attached primarypiston 4 a required distance from the current off end stop. Theelectronic monitor unit 10 calculates the duration of current pulse thatwill produce the required movement from the empirical relationship andapplies this current pulse to the solenoid coil 3. The magnetic field inthe solenoid coil 3 moves the plunger 2 towards it and drives theprimary piston 4 down the primary cylinder 5, also moving the secondarypiston 22 down the secondary cylinder 23.

As the primary piston 4 moves down the primary cylinder 5 the pressureof the fuel in the primary pressurisation chamber 21 will increase untilit exceeds the fuel pressure in the atomisation chamber 12 by theopening pressure of the primary outlet valve 7 causing this valve toopen. The fuel pressure in the atomisation chamber 12 will then increaseto such a level as to open the atomisation pintle valve 13 bycompressing the atomisation valve spring 14, this pressure typicallybeing at least 3 bar to ensure good atomisation. The fuel will then flowthrough the primary outlet valve 7 and through the atomisation valve 13into the inlet manifold 20.

During this injection stroke the pressure of the fuel in the secondarypressurisation chamber 26 will drop until the pressure of the fuel inthe fuel inlet line 17 will exceed it by the opening pressure of thesecondary inlet valve 24 causing that valve to open. Fuel will then bedrawn into the secondary pressurisation chamber 26. The secondary inletvalve 24 is optimised to cause the minimum possible pressure drop duringthis secondary inlet phase, but even so there may be a tendency forbubbles to form within the secondary pressurisation chamber 26. Howeverany bubbles formed will not cause problems unless they are subsequentlydrawn into the primary high pressure pump assembly. The secondarypressurisation chamber 26 is thus designed so that any bubbles formedtend to float to the top of the secondary pressurisation chamber 26where they are as close as possible to the secondary outlet valve 25 andas far as possible away from the primary inlet valve 8. They will thenbe purged out of the secondary pressurisation chamber 26 into the endstop chamber 6 during the secondary low pressure pump outlet stroke.

The plunger 2 will continue to accelerate whilst current is passedthrough the solenoid coil 3, reaching maximum velocity at the end of thecurrent pulse. The plunger 2 will then continue to move in the samedirection under its own momentum but will be decelerating under theforce of the return spring 11, the force of the fuel pressure in theprimary pressurisation chamber 21 on the end of the primary piston 4 andthe force of the fuel depression in the secondary pressurisation chamber26 on the secondary piston 22. During this deceleration phase theelectronic monitor unit 10 will use the position measurement circuitrycontained within it to continuously monitor the position of the plunger2 by monitoring the electrical characteristics of the solenoid coil 3.The electronic monitor unit 10 will note the maximum displacementindicated by the position measurement circuitry which will occur whenthe force of the return spring 11 and primary and secondary fuelpressures bring the plunger 2 to a halt at the apogee of its travel. Theprimary outlet valve 7 and atomisation valve 13 will then close and thereturn spring 11 will then start to accelerate the plunger 2 backtowards its current off end stop 15.

At the start of this return stroke the secondary inlet valve 24 willclose and the pressure of the fuel in the secondary pressurisationchamber 26 will rise until it exceeds the pressure of the fuel in theend stop chamber 6 by the opening pressure of the secondary outlet valve25 causing this valve to open. The opening pressure of the secondaryoutlet valve 25 will be set to a value that prevents any significantbubble formation during this phase of operation. This will typically bebetween 0.2 and 0.4 bar. The pressure of the fuel in the primarypressurisation chamber 21 will fall until the pressure of the fuel inthe secondary pressurisation chamber 26 exceeds the pressure of the fuelin the primary pressurisation chamber 21 by the opening pressure of theprimary inlet valve 8, the primary inlet valve 8 will then open and fuelwill be forced into the primary pressurisation chamber 21 from thesecondary pressurisation chamber 26. Because the diameter of thesecondary piston 22 is considerably greater than that of the primarypiston 4 only a proportion of the fuel pumped by the secondary piston 22will be drawn into the primary pressurisation chamber 21, the excessbeing forced through the secondary outlet valve 25 and back to the fueltank via the end stop chamber 6, and the cooling passages around theplunger 2 and solenoid coil 3.

Once the plunger 2 reaches its current off end stop 15 the electronicmonitor unit 10 will note the output of the positional feedback systemwith the plunger 2 resting on its current off end stop 15. It will thensubtract this offset reading from the positional feedback system outputthat was noted with the plunger 2 at its maximum deflection to give thetotal movement of the plunger 2 and attached primary piston 4. Thismovement will be compared to the required distance that it wasoriginally intended to move the plunger 2 and attached primary piston 4.Any error term will be used to provide feedback to correct the empiricalrelationship between solenoid current pulse duration and piston movementfor future injection operations. For example if the plunger 2 andattached primary piston 4 were moved slightly less than the intendeddistance the solenoid pulse tables will be increased so that the nexttime an injection operation is carried out the solenoid coil 3 will beenergised for slightly longer.

A self calibration operation may be carried out at intervals tocalibrate the positional feedback system. The electronic monitor unit 10will first note the output of the positional feedback system with thesolenoid coil 3 switched off and the plunger 2 in contact with itscurrent off end stop 15. FIG. 1 shows the plunger 2 in this position.The electronic monitor unit 10 will then switch the solenoid coil 3 onfor a comparatively long period to allow the plunger 2 to come intocontact with its current on end stop 16. FIG. 2 shows the plunger 2 inthis position. The electronic monitor unit 10 will then note the outputof the positional feedback system. The electronic monitor unit 10 nowhas two output readings from the positional feedback system a knowndistance apart, hence it can calibrate the scaling factor of thepositional feedback system.

An important advantage of the apparatus shown is that it does not need aseparate high pressure fuel supply. This saves cost, weight and space.These savings are particularly important on single cylinder applicationssuch as low capacity motorbikes where the high pressure fuel systemrepresents a very large overhead. The high pressure fuel system is alsopotentially hazardous and eliminating it presents an important safetyadvantage. Again this is particularly important for two wheeledapplications where the operator is not as remote from the fuel system asthey are in an automobile.

An important advantage of the apparatus shown is that it meters the fueldelivered from first principles, that is by a simple volumetriccalculation related to the distance the actuator moving magnet assemblyis moved. It is thus not subject to some of the fuel delivery errorsthat a conventional electronic fuel injection system is subject to dueto variations in injector orifice diameter, and injector orificepressure differential.

An important advantage of the apparatus shown is that higher injectorpressures are both feasible and safe. Higher injector pressures lead toimproved atomisation which leads to improved combustion efficiency andreduced emissions. By providing a stronger drive signal and higheratomisation valve opening pressures injection pressures of 18 to 40 barare readily obtainable using actuators currently available. Providing aninjection pressure of 40 bar with an external fuel pump and regulatorwould be both expensive due to the heavy duty pumps and fuel linerequired, and potentially hazardous.

An important advantage of the apparatus shown is that it utilises asingle solenoid coil to both drive the plunger, and to monitor theposition of that plunger. This means the device is simple inconstruction, cheap to produce, and requires fewer electricalconnections to the outside world.

An important advantage of the apparatus shown is that the integralsecondary low pressure pump will suppress the production of bubbleswithin the fuel during the inlet stroke of the primary high pressurepump. During the inlet stroke of the primary high pressure pump the fuelwill be subjected to a reduction in pressure as it flows through theprimary inlet valve. When injecting a fuel containing constituents withlow boiling points, for example gasoline, there is a tendency for thisreduction in pressure to cause bubbles to form within the fuel in theprimary pressurisation chamber. Such bubbles will cause errors in themetering of the fuel. The integral secondary low pressure pumppressurises the fuel during the inlet stroke of the primary highpressure pump limiting bubble formation. The secondary low pressure pumpalso circulates fuel through the injector purging any bubbles that doform within the secondary pressurisation chamber.

The current invention may be operated without an integral secondary lowpressure pump. If this is the case the current invention may require anexternal secondary low pressure pump to provide a circulatory flowthrough the injector. This external secondary low pressure pump bothprovides a small positive pressure to reduce the tendency for bubbleformation, and will also flush through the injector any bubbles that doform.

The above embodiments all show injectors which are injecting fuel intothe inlet manifold of an engine. The apparatus shown could also be usedto inject directly into the engine cylinder. This can reduce theemissions from an engine, as the injection can be carried out after theexhaust valve has closed, preventing fuel short circuiting between theinlet and exhaust valves. This is a particular problem on two strokeengines and four stroke engines with large valve overlaps. The apparatusshown has two advantages over conventional systems for this application.Firstly as has been outlined above it can readily generate higherinjection pressures. This is necessary for direct cylinder injection toovercome the pressure in the cylinder which may be higher than that inthe manifold. Secondly the amount of fuel dispensed is metered fromfirst principles and will not be affected by the variations in cylinderpressure which will occur as the piston moves in the bore on thecompression stroke. If a conventional fuel injection system is used,where fuel flow is determined by the pressure drop across the injectororifice, these pressure variations make metering the fuel complex andsubject to error.

1. A fuel injector characterised in that the fuel injector comprises anelectrical actuator comprising a solenoid coil and a movable plunger; apump assembly comprising piston means being movable by the electricalactuator; and monitoring means for controlling the amount of fueldispensed, the arrangement being such that, in use, fuel is dispensedfrom the fuel injector by the movement of the plunger and the pistonmeans, and the monitoring means analyses the change in the electricalproperties of said solenoid coil caused by the change in the position ofthe plunger within said solenoid coil to ascertain the position of theplunger.
 2. A fuel injector as claimed in claim 1, wherein the analysisof the position of the plunger is used by monitoring means to controlthe amount of fuel being dispensed.
 3. A fuel injector as claimed inclaim 1 or claim 2, wherein in use the movement of the plunger of theelectrical actuator and the piston means over a predetermined distancewill dispense a predetermined amount of fuel.
 4. A fuel injector asclaimed in claim 1, wherein the actuator comprises a fixed solenoid coiland a return spring for the movable plunger.
 5. A fuel injector asclaimed in claim 1, wherein the monitoring means comprises positionmeasurement circuitry the arrangement being such that in use thesolenoid coil provides motive force to the plunger and to the pistonmeans, and the solenoid coil provides positional feedback information ofthe position of the plunger to the position measurement circuitry of themonitoring means.
 6. A fuel injector as claimed in claim 2, wherein thesolenoid coil comprises a single winding with two external electricalconnections.
 7. A fuel injector as claimed in claim 2, wherein the fuelinjector comprises fuel atomisation means, the arrangement being suchthat in use fuel is forced through the atomisation means by the movementof the actuator; the actuator generating sufficient force to pressurisethe fuel to a level which will obtain satisfactory atomisation.
 8. Afuel injector as claimed in claim 7, wherein the atomisation means is anatomisation orifice.
 9. A fuel injector means as claimed in claim 4,wherein the actuator comprises an end stop, the arrangement being suchthat with no current flowing through the solenoid coil the rectum springmoves the plunger in a direction towards the end stop.
 10. A fuelinjector means as claimed in claim 9, wherein when current is passedthrough the solenoid coil the plunger moves in a direction away from theend stop and the piston means dispenses fuel from the fuel injector. 11.A fuel injector means as claimed in claim 9, wherein in use duringnormal operation the monitoring means provides electrical powers to thesolenoid coil for short periods of time such that the distance theplunger moves away from the end stop is controlled by the duration andlevel of the pulse of electrical current that is applied to the solenoidcoil.
 12. A fuel injector means as claimed in claim 11, wherein themonitoring means controls and varies the amount of fuel dispensed byvarying the duration of the pulse of electrical current that is appliedto the solenoid coil, the arrangement being such that in use the greaterthe duration of the current pulse that is applied to the solenoid coilthe greater the distance moved by the plunger and the piston means andthe greater the amount of fuel dispensed.
 13. A fuel injector means asclaimed in claim 12, wherein the monitoring means controls and variesthe amount of fuel dispensed by varying the level of current that ispassed through the solenoid coil during a current pulse, the arrangementbeing such that in use the higher the current level that is appliedduring the current pulse the greater the distance moved by the plungerand the piston means and the greater the mount of fuel dispensed.
 14. Afuel injector means as claimed in claim 13, wherein the monitoring meanscontrols the amount of fuel dispensed by varying both the duration ofthe current pulse and the level of current within that pulse.
 15. A fuelinjector means as claimed in claim 13, wherein in use the monitoringmeans applies a current pulse of a predetermined duration and level tothe solenoid coil and once the current pulse has ended the monitoringmeans monitors the position of the plunger as it continues to move inthe same direction due to its own momentum, and as the plunger slows toa halt due to the forces exerted upon it by the return spring and thefuel pressure on the end of the piston means, the monitoring means notesthe maximum deflection reached by the plunger.
 16. A fuel injector meansas claimed in claim 15, wherein the monitoring means uses themeasurement of the maximum deflection reached by the plunger toestablish an empirical relationship between the duration and level ofthe current pulse, and the distance moved by the plunger.
 17. A fuelinjector means as claimed in claim 16, wherein the monitoring means usesthe empirically derived relationship between the duration and level ofthe current pulse, and the distance moved by the plunger, to predict theduration and level of the current pulse that needs to be applied to thesolenoid coil to move the plunger the distance that will dispense arequired amount of fuel from the fuel injector.
 18. A fuel injectormeans as claimed in claim 17, wherein in use for any given fueldispensing stroke of the piston means the monitoring means notes theactual distance moved by the plunger and then compares this distance toa predetermined distance the plunger was required to move in order todeliver predetermined correct amount of fuel; the difference between thepredetermined and actual distance provides an error term that is fedinto a feedback algorithm that modifies the empirical relationshipbetween the duration and level of the current pulse, and the distancemoved by the plunger.
 19. A fuel injector means as claimed in claim 9,wherein in use when no current is flowing in the solenoid coil and thereturn spring has returned the plunger to the end stop, the monitoringmeans uses the value of the electrical properties of the solenoid coilat this point as an offset reference value.
 20. A fuel injector means asclaimed in claim 19, wherein during a fuel dispensing operation theoffset reference value obtained when the plunger is resting on the endstop is subtracted from all subsequent output readings from the solenoidcoil to enable the distance moved by the plunger from the end stop to becalculated.
 21. A fuel injector as claimed in claim 20, wherein the fuelinjector comprises a second end stop at the other end of the travel ofthe plunger, the arrangement being such that the plunger rests againstthe second end stop if a current is passed through the solenoid coil forlong periods of time.
 22. A fuel injector as claimed in claim 21,wherein the fuel injector is capable of delivering any required amountof fuel for the engine with a stroke of the plunger and piston meansthat is less than the maximum stroke limit of the injector as dictatedbyte second end stop.
 23. A fuel injector as claimed in claim 22,wherein at least one of the strokes of the plunger is over the fullstroke distance of the plunger as dictated by the second end stop, thisenabling the positional feedback system calibration operation to becarried out more frequently.
 24. A fuel injector as claimed in claim 21,wherein the fuel injector delivers the fuel in two strokes of theplunger and piston means, at least one of those strokes being over thefull stroke distance of the plunger as dictated by the second end stop,this enabling the positional feedback system calibration operation to becarried out more frequently.
 25. A fuel injector as claimed in claim 1,wherein the pump assembly comprises two pumps; a primary high pressurepump and a secondary low pressure pump, the arrangement being such that,in use, the primary high pressure pump transfer the fuel into an engineand the secondary low pressure pump generates positive pressure duringan inlet stroke of the primary high pressure pump reducing the tendencyfor the formation of bubbles within the primary high pressure pump. 26.A fuel injector as claimed in claim 25, wherein the primary highpressure pump comprises a primary piston reciprocatable within a primarycylinder chamber comprising a primary inlet valve and a primary outletvalve.
 27. A fuel injector as claimed in claim 25, wherein the secondarylow pressure pump comprises a secondary piston reciprocatable within asecondary cylinder chamber comprising a secondary inlet valve and asecondary outlet valve.
 28. A fuel injector as claimed in claim 27,wherein the primary piston and the secondary piston are substantiallycoaxial.
 29. A fuel injector as claimed in claim 28, wherein thediameter of the primary piston is less than the diameter of thesecondary piston.
 30. A fuel injector as claimed in claim 29, wherein inuse the inlet stroke of the primary high pressure pump occurs during theoutlet stroke of the secondary low pressure pump; during the outletstroke of the secondary low pressure pump the pressure of the fuel inthe secondary cylinder chamber of the secondary low pressure pump beingraised to a value dictated by the opening pressure of the secondaryoutlet valve of the secondary low pressure pump; the primary inlet valveof the primary high pressure pump being in fluid communication with thesecondary cylinder chamber of the secondary low pressure pump and thepressurized fuel therein.
 31. A fuel injector as claimed in claim 25,wherein in use a predetermined plunger movement the volume of fueldisplaced by the secondary low pressure pump is considerably greaterthan the volume of fuel displaced by the primary high pressure pump; asa result during the inlet stroke of the primary high pressure pump andoutlet stroke of the secondary low pressure pump only a part of the fuelbeing pumped by the secondary low pressure pump is drawn into theprimary high pressure pump; the excess fuel pumped by the secondary lowpressure pump passing through the secondary outlet valve of thesecondary low pressure pump so allowing for the generation of netpositive pressure during the inlet stroke of the primary high pressurepump as dictated by the opening pressure of the secondary outlet valveof the secondary low pressure pump.
 32. A fuel injector as claimed inclaim 25, wherein the fuel injector the primary inlet valve of theprimary high pressure pump may be incorporated within the plunger.
 33. Amethod fur detecting the position of a movable plunger of a fuelinjector comprising an electrical actuator comprising a solenoid coiland the movable plunger; a pump assembly comprising piston means beingmovable by the electrical actuator and monitoring means fur controllingthe amount of fuel dispensed characterised in that the method comprisesfirst moving the plunger and the piston means using the solenoid coil,and then monitoring the change in the electrical properties of thesolenoid coil caused by the change in the position of the plunger withinthe solenoid coil to provide the position of the plunger.
 34. A methodfor detecting the position of a movable plunger as claimed in claim 33,wherein the method comprises the analysis of the position of the plungerby the monitoring means and the use of the analysis for controlling theamount of fuel being dispensed by the fuel injector.